Stromal cells provide structural support for malignant cells, modulate the tumor microenvironment, and influence phenotypic behavior as well as the aggressiveness of the malignancy. In response, the tumor provides growth factors, cytokines, and cellular signals that continually initiate new stromal reactions and recruit new cells into the microenvironment to further support tumor growth. It is not fully understood how stroma influences the neoplastic cells, but there is evidence for involvement of soluble paracrine factors, extracellular matrix formation, and direct cell-to-cell interaction. Therefore, it might be possible to manipulate the tissue stroma cells and thereby interfere with the stroma-tumor interactions for therapeutic benefit. The prerequisite for this approach is that stromal cells be accessible to therapeutic manipulation. We have previously demonstrated that bone marrow-derived mesenchymal stem cells (MSC) integrate into solid tumors as stromal elements and contribute to the development of tumors. Importantly, MSC are precursors of structural and supportive tissues and have been implicated in the repair of damaged tissues and in wound healing. Of interest is that the tumor microenvironment appears to exhibit cytokine profiles and cellular signals similar to those characteristics of wounded or damaged tissues. Given this, we hypothesized that MSC would home to and selectively proliferate in the tumor microenvironment and that gene-modified MSC could be used as cellular vehicles to deliver gene products into tumors. Preliminary data suggests that MSC home to and participate in tumor stroma formation in both subcutaneous and metastatic tumor xenografts in mice. Additionally, once homed to tumor beds, MSC proliferate rapidly and integrate. Migration assays have identified apoptotic cells as a potent attractant of MSC in vitro. Our proposed studies aim at understanding the factors that influence MSC homing and selective proliferation in the tumor microenvironment. Additionally, our goals focus on optimizing the cellular delivery of therapeutic genes into the stroma of metastatic and subcutaneous tumor xenografts. Specific Aim 1: To determine the biodistribution and selective proliferation of intravenously administered MSC in the stromal microenvironment, to identify cellular mediators that may enhance the selective proliferation and engraftment of MSC, and determine the phenotype and fate of MSC that have engrafted and proliferated in the tumor microenvironment. Specific Aim 2: To investigate the tumor targeting ability and normal distribution of MSC in vivo utilizing noninvasive imaging techniques. Specific Aim 3: To determine which of several viral gene delivery systems + MSC results in optimal tumor growth inhibition and cell killing. Additionally, to determine the effect of MSC + vector specifically targeted to breast cancer or metastatic melanoma in vivo, on the growth of tumors in immunocompetent and immunodeficient mice.